Radiation therapy of patients for cancer treatment takes place in radiation rooms. Tumors are thereby irradiated with ionizing radiation by means of radiation devices. The correct positioning of the patient is decisive so that the ionizing radiation hits the tumor optimally. In a computed tomography (CT) room that is separate from the radiation room, the area to be irradiated is localized based on CT images and markings are applied to the patient's body, based on which the patient is then positioned in the radiation room. For this, so-called room lasers are arranged in the radiation room. Room lasers are laser projectors arranged permanently on the ceiling or wall in the radiation room, which generate one or two light arrays. At least three room lasers may be installed, which are pointed at the isocenter of the radiation device. Based on the markings applied to the patient's body and by means of the room lasers, the patient is aligned for the radiation through a suitable moving of a patient table. In particular, the markings applied to the patient's body are brought to overlap with the laser crosses aligned with the isocenter of the radiation device. For the purpose of positioning, only a small area of the respectively projected laser lines around the laser cross is used. Due to the expansion of the light arrays, other objects in the radiation room besides the patient are normally also illuminated by the room lasers.
A device for monitoring the position of a patient receiving radiation is known from DE 103 42 202 A1, in which two or more distance measuring devices measure the distance to respectively one point on the skin of the patient. An evaluation apparatus determines from at least two distance values whether the position of the patient has changed with respect to an initial position. So-called off-axis triangulation can be used for the distance measurement. Another device for capturing the position of an object located in a radiation room is known from DE 297 24 767 U1. A collision of components of the medical apparatus, for example a radiation transmitter, with other objects located in the room should thereby be avoided. A triangulating 3D technique can be used. This known device is also structurally complex since the light transmitters and cameras used for the measurements must also be housed in the radiation room.